Digital cellular communication systems are known. Such systems are, typically, comprised of a number of cells, each having a service coverage area, and a number of cellular telephones (communication units) (also sometimes referred to hereinafter as mobile stations or MSs). The service coverage areas of adjacent cells are typically arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a communication unit receiving service from one cell may be handed off to an adjacent cell with no interruption in service. The Groupe Special Mobile (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI) is an example of just such a system.
The GSM system is a TDM/TDMA system providing eight full duplex signal paths (8 TDM slots per TDM frame) on each radio channel. A single, primary radio channel assigned to a base transceiver station (BTS) located at a base site within a cell, by virtue of its being time multiplexed, can support up to seven full rate duplex traffic users (speech or data) in addition to a multiplexed common control channel within the eight TDM slots.
Exchanges of paging and setup control information within GSM between MSs and BTSs typically occurs on the common control channel (CCCH) which occupies at least one slot of a primary channel of the BTS. Transmitted by the BTS on the CCCH are distinctive identification signals as well as synchronization and timing information common to all other frequencies and slots of the BTS. CCCH information allows an MS to differentiate between primary and non-primary channels.
Upon activation, an MS scans a pre-programmed spectrum in search of CCCH identification signals transmitted from nearby BTSs. Upon detecting a CCCH identification signal, the communication unit measures a signal quality factor (such as signal strength) of the identification signal as a means of determining relative proximity of the BTS. Upon completing the scan of frequencies within the spectrum, the MS generally selects the BTS providing the largest relative signal quality factor as a serving BTS. Upon identifying, and locking onto a suitably strong signal, the communication unit monitors the selected CCCH for incoming calls. While monitoring the serving BTS, the MS receives an adjacent base site frequency list on the CCCH of the serving BTS. The set of frequencies identifies the spectral location of primary channels of BTSs, adjacent the serving BTS. Limitation of the set of frequencies to BTSs adjacent the serving BTS reduces the time period required to measure, and transfer to a serving BTS the signal strength values of BTSs that are presumably the best handover candidates. Limitation of the set of frequencies to adjacent BTSs also reduces the possibility of handover to a distant BTS with a high signal strength value caused by signal propagation anomalies.
During normal operation (including during active calls), the MS monitors for, identifies, and measures a received signal strength indication (RSSI) of primary channels of nearby BTSs. The MS detects and measures the RSSI of nearby BTSs by reference to the frequency list communicated to the MS by the serving BTS. Upon detection of a signal of a nearby BTS, the MS also decodes an ID of the nearby BTS.
If involved in an active call, the MS relays measurement information back to the base site on an associated signaling channel. Through such a process, it is possible for the MS to maintain an association with the most appropriate (proximate) BTS. The process may entail an autonomous switching by the MS to a different BTS, causing perhaps a re-registration by the MS with the system indicating that such a switch has occurred. Alternatively, during an active communication exchange, the MS may be commanded by the system to handover to a more appropriate BTS. Handover based upon information provided to the BTS by the MS is commonly referred to as a mobile assisted handover (MAHO).
Under GSM, a decision to handoff a communication unit to a target BTS may be based upon a power budget expression (see GSM Recommendation 5.08). The power budget expression provides a method of comparing a path loss between an MS and serving cell with a path loss between the MS and a potential handoff target cell.
Under GSM, handover may also be desirable when the MS exceeds a specified distance from a serving BTS. Handoff may be desirable in such case to minimize effective cell size and to insure that an MS is served from the nearest BTS. Other handover causes, as specified in GSM recommendation 5.08, include handover for reason of RXQUAL (high bit-error-rate threshold), and handover for reason of RXLEV (down link threshold or uplink threshold).
Where the decision to handoff is based on distance, the parameter that may be used as an indication of distance is timing advance. Timing advance is a parameter measured by a BTS based upon round-trip signal delay of a signal transmitted from the serving BTS to the MS and back to the BTS. The measured value is then used to adjust the timing of the MS to ensure that transmissions from an MS arrive at a BTS within the TDM slot assigned to the MS.
While the prior art handover algorithm has worked well, problems may be experienced in target selection. Under GSM, potential targets are determined from RSSI values measured by the MS. Potential targets, on the other hand, are limited to BTSs adjacent the serving BTS (as determined by the frequency list communicated to the MS). If handover is deferred because a target BTS is operating at capacity, then the MS may move past adjacent BTSs into the service areas of non-adjacent BTSs. As the MS continues to receive service through the original serving BTS the MS may not detect nearby non-adjacent BTSs (as in the case of a fast mobile moving through micro cells). If the MS moves too far from the serving BTS before a channel becomes available in an adjacent BTS then the MS may be dropped. Even if not dropped, an MS operating relatively far from a serving BTS, presumably at full power, may create a source of interference sufficient to cause dropped calls in the case of other MSs.
When a channel becomes available in a target BTS (adjacent the original serving BTS) a handover may take place to the target BTS. At that time, a new frequency list may be transferred to the MS. The MS may now measure an RSSI value of a target BTS adjacent the new serving BTS, and a further handover may take place that finally places the MS in the most proximate cell. The result of all of this handover activity is a ratcheting of the MS from one cell to another; from the original serving BTS, to an adjacent BTS, to a proximate BTS. Ratcheting not only consumes valuable control resources of a communication system but also reduces the availability of traffic channels as intermediate BTSs alternately reserve, and then release, traffic channels as the MS ratchets to a proximate BTS.
Ratcheting may also occur as a result of signal anomalies where obstructions or other local signal propagation characteristics cause a MS to pass through adjacent BTS service areas without handover. Because of the problem associated with ratcheting, a need exists for a better method of identifying proximate BTSs by MSs.